Field of the Invention
The present invention pertains to the field of optics. In particular, it relates to a lens design having a modifiable refractive index that can be maintained after applying and removing an external agent, but also can be remodified to other values.
Description of the Related Art
Ophthalmic lenses are designed to provide corrective optical power to improve visual acuity, correct for aberrations or optical defects in the eye and enhance quality of life and visual performance. Passive (fixed power) ophthalmic lenses have been known for years, and recent technical advancements allow them to correct errors in vision more precisely. However, the needed visual correction for an individual changes over time, due to age-related physiological changes, such as the onset and progression of presbyopia. In addition, an individual's needed correction may change due to stress, illness, accident, medical treatments, environmental conditions, and personal preferences. For example, if one is trying to read very fine print, or work with minute intricate machinery, one may have a need for greater magnified power. In another instance, progressive lenses typically have a near-vision reading area in the lower portion of the lens, for hand-held reading. However, if one is viewing a display mounted at the top of an eyeglass frame or a helmet, additional power for such near focus may be needed in the upper region of the viewing area. Thus, there is a need for active lenses that could allow optical power to be varied either in its total value and/or in the power distribution (placement) on the lens.
Certain methods have recently been proposed or attempted to address this need. The use of electro-optical materials for the realization of variable refractive power spectacle lenses is known. For a spectacle lens, these systems comprise an embedded set of electrodes, power connections and electro-optical material that maintains the spectacle lens refraction power as long as the incorporated battery actively supplies the necessary electrical power. These systems are constructed with active optical materials wherein an external agent (i.e. an electrical field) can change an optical property of the lens; in this case, the external agent changes the refractive index and therefore modifies the lens refractive optical power. The disadvantage is that the external agent must be present to hold the desired value of the refractive index. Typically this means the battery, or at least its connections, must be present on the spectacles, resulting in a heavier, more cumbersome or less stylish design.
Another example of variable refractive power systems are fluid-filled lenses. They consist of back and front pieces held apart by a sealing edge ring that form a flexible chamber. The flexible chamber is filled with liquid and hydrostatic pressure changes the shape of the chamber/interface. These systems react to an external agent (e.g., hydrostatic pressure) and the pressure can be maintained at a given value by sealing the chamber. If the material is unlocked (i.e., the pressure on the liquid is released), the system can become active again; thus the system is rewritable, but not “freezable” because the pressure must be maintained to keep the desired values.
For a freezable optical system, the refractive power can be locked at a given refractive state, and once the source of energy or influence that has caused the change is removed, the system remains in that locked state.
While fluid-filled lenses are an interesting development, they have certain drawbacks. The possible surface configurations are limited to the set of elastic solutions of a deformed clamped membrane under uniform hydrostatic pressure. This limits selectable power variations to a change in the total surface, or only a limiting, predetermined portion of the surface that is allowed to deform to create a different power. Thus, fewer prescriptions and fewer personalization options are accommodated. In addition, to allow predictable control of the power, a lens shape must be used that will uniformly distribute the liquid and its pressure. This typically limits the lens shape to only one configuration: round lenses. This may not be the preferred shape for many individuals. In addition, the fluid-filled lens requires double cavities, pneumatic mechanisms, delivery tubes or other components, which are often too bulky for modern and fashionable ophthalmic lenses.
The electro-optical and fluid-filled refractive power systems are rewritable, in the sense that by means of the external agent (e.g., electrostatic field or hydrostatic pressure) the refractive power distribution first can be configured, and then can be changed by altering the influence of the external agent. The electro-optical system requires a continuously maintained influence of the external agent to maintain the chosen optical power. The fluid-filled lenses require continuous pressure to maintain their chosen optical power, but have some notable limitations in their use.
In addition to the drawbacks mentioned previously, both of these systems would have significant limitations or introduce practical concerns if implemented for ophthalmic lenses other than for spectacles, goggles or head-mounted frames. While electrical connections or liquid lines can be imagined for contact lenses or intraocular lenses, their engineering, validation, maintenance and replacement will entail considerable extra work, care, investment and significant risk reduction. There is also the practical concern of whether the device will be comfortable or consistently functional for the wearer; often eyes can become extremely sensitized when even minute additional bodies are placed in the eye's physical structure. Therefore, it would be preferable to find other ways to create a rewritable lens that do not necessarily entail external connections, either intermittent or continuous.
On the other hand, non-rewritable lenses (writable only once) made with passive materials are known. For example, passive progressive power lenses have been proposed by Fischer by forming lenses (or adding material to existing lens substrates) using radiation-polymerizable material that exhibit a diffusion gradient. The desired spatial power distribution is achieved by means of a spatial pattern of the polymer-curing UV radiation; a spatial refractive index distribution is generated as a function of the polymerization degree. In this case, the generated spectacle lens is writable but not rewritable, in the sense that once the external agent (the curing UV radiation) ends, the power distribution of the lens becomes permanent. One option to ameliorate this limitation is to apply the UV curing technique to only one surface of a lens blank and manufacture semi-finished lenses. This allows further personalization and a wider range of prescriptions to be met by modifying the other surface using standard digital surfacing methods.
Other approaches to modify optical properties once (writable) by adding or combining different lens materials or embedded substances have been described in various publications, for example, in Hudelist, et al., “Design and fabrication of nano-structured gradient index microlenses,” Opt. Express, vol. 17, no. 5, pp. 3255-3263, March 2009, and U.S. Pat. Nos. 5,861,934, 8,240,849 B2 and 8,625,198 B2.
The idea of a rewritable and freezable reflective lens is presented by Cheng, et al., “Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals,” Opt. Express, vol. 15, no. 21, pp. 14078-14085, 17 Oct. 2007. In this work, the authors describe a rewritable reflective, or polarization-dependent transreflective, Fresnel lens based on the use of dye-doped cholesteric Liquid Crystal (LC). The lens is written or formed by the LC photoalignment effect using coherent polarized light. The formed reflective Fresnel lens is freezable because once formed, the Fresnel lens structure persists without an external agent. However, the liquid-crystal orientation of the formed lens is thermally erasable and could be rewritable to another reflective or diffractive Fresnel structure with another application of the external agent, in this case another illumination pattern directed at the LC layer. Depending on the incidence of light, this can produce reflective or dispersive (scattering) regions. The properties of this type of lens will depend strongly on wavelength, polarization and incidence angle, which can limit its practical applications. A reflective Fresnel lens would not be appropriate replacement for a transmissive spectacle lens; even diffractive Fresnel lenses have severe limitations for ophthalmic use due to their inherent scattering.
Despite these significant drawbacks, new and innovative applications of liquid crystal systems may be explored.
Bistable LC materials are known from the literature to have two possible states of polarization, each one with an associated refractive index. These materials are generally considered active because they respond to the application of an electrical field, but they are also freezable because the local polarization state remains fixed once the electrical field is removed. Such systems have been used for thin film and flat panel displays. These materials might be adapted for more innovative and complex ophthalmic applications using rewritable and freezable binary optical systems.
The previous optical systems do not combine all the properties or design freedom desired for ophthalmic lens applications. For example, they are not both rewritable and freezable to allow unencumbered and adaptable lenses. Other prior approaches do not provide enough degrees of freedom to generate arbitrary refractive power distributions, an essential feature to create optimal, personalized lenses. Additional improvements and innovations are warranted.